Applying nitride semiconductors to high-withstand-voltage and high-power semiconductor devices by utilizing their characteristics such as a high saturation electron velocity and a wide band gap has been considered. For example, GaN being a nitride semiconductor has a band gap of 3.4 eV, which is wider than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV), and has high breakdown electric field intensity. This makes GaN very promising as a material of semiconductor devices for power supply realizing a high voltage operation and a high power.
Many reports have been made on field-effect transistors, in particular, HEMTs (High Electron Mobility Transistors) as semiconductor devices using nitride semiconductors. For example, among GaN-based HEMTs (GaN-HEMTs), an AlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer has been drawing attention. In the AlGaN/GaN HEMT, a distortion ascribable to a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectric polarization caused by the distortion and spontaneous polarization of AlGaN, high-concentration two-dimensional electron gas (2DEG) is obtained. Therefore, the AlGaN/GaN HEMT is expected as a high-efficiency switch element or a high-withstand-voltage power device for electric vehicles and the like.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2012-134345
A GaN-HEM is expected to have a high withstand voltage of, for example, 400 V or more. There has been a conventional concern about a breakdown of a gate electrode when a high voltage as above is applied to the GaN-HEMT. In recent years, it has come to be found out that a breakdown occurs also in a drain electrode in ohmic-contact with a nitride semiconductor. The breakdown of the drain electrode is ascribable to electric field concentration occurring on an end of the drain electrode. Consequently, electrons and holes are simultaneously generated by an avalanche effect, and the electrons and the holes further continuously cause the cumulative generation of electrons and holes, so that a current rapidly increases to cause the breakdown in the drain electrode. It has been confirmed that this breakdown of the drain electrode occurs both when a recess is formed in the nitride semiconductor and the drain electrode is formed in the recess and when the drain electrode is formed on the nitride semiconductor without a recess being formed.